Several processes have been used commercially or have been proposed to separate carbon dioxide, sulfide gases and hydrocarbons from carbonaceous off-gases.
The Ryan/Holmes process which is disclosed in U.S. Pat. Nos. 4,293,322; 4,318,723; 4,350,511 and 4,383,841 is an extractive distillation process specifically for high carbon dioxide gas processing which utilizes a natural gas liquid column additive to separate the carbon dioxide from methane, carbon dioxide from ethane, and carbon dioxide from hydrogen sulfide. Incremental energy costs for higher propane recovery, e.g. above 30%, exceed recovered C.sub.3 incremental revenues.
Carbon dioxide is separated from light hydrocarbons by permeation through a suitable membrane in a process described in "Membranes for Natural Gas Sweetening and CO.sub.2 Enrichment", Chemical Engineering Progress, October 1982, Pages 38-43. Carbon dioxide permeate is produced at low pressure and recompression is very energy intensive. Both the CO.sub.2 recompressor and the membranes are high in capital cost.
Various absorbent solvents have also been utilized to absorb CO.sub.2 from carbonaceous off-gases. In these processes, the carbon dioxide is recovered from the solvent at low pressure, such processes being generally energy intensive in both thermal and compression energy and requiring high capital expenditures. Typical solvent absorbent processes are described in U.S. Pat. Nos. 4,158,467 and 4,270,937.
U.S. Pat. No. 4,417,449 discloses a process in which carbonaceous off-gases are separated in a distillation column into an overhead fuel gas stream, a carbon dioxide-sulfide liquid sidestream and a heavy hydrocarbon-carbon dioxide liquid bottom fraction. The bottom fraction is further separated into a recycled carbon dioxide stream and a product natural gas liquid stream. The process requires relatively low energy and capital.